This invention relates to antenna systems to receive signals from Global Positioning System (GPS) satellites and, more particularly, to antenna systems capable of highly accurately determining errors in a current GPS-indicated position of a fixed location of known latitude, longitude and elevation.
Implementation of GPS-based precision aircraft approach and landing systems necessitates provision for currently verifying the integrity of the system. Particularly for systems capable of enabling aircraft landings at airports under zero visibility atmospheric conditions, small errors in the GPS-indicated position of an aircraft relative to the runway surface may be critical to the accomplishment of a safe landing. Variations in the integrity of a GPS-indicated position are inherent due to variations and errors which may occur in the transmission of the signals from GPS satellites due to atmospheric and other conditions.
Pursuant to the invention, an integrity monitor antenna system may be employed to enable derivation of error data on a current basis representing differentials between a GPS-indicated position and the actual fixed position of known latitude, longitude and altitude of the antenna system. However, it is critical that an antenna employed for this purpose be capable of receiving GPS signals without itself inherently introducing errors which may adversely affect the derivation of a GPS-indicated position by use of the received GPS signals. Thus, if the antenna itself is subject to signal reception errors, it will not be possible to accurately evaluate actual errors inherent in the GPS-indicated position developed from the received GPS signals.
In this context, antenna susceptibility to effects of reflected multipath GPS signals is of major significance. Extreme ground multipath mitigation, defined for this purpose as 40 to 50 dB suppression of reflected GPS ground signals, is desirable. Consistent with this, it is desirable that with such enhanced multipath performance the antenna employed be capable of achieving a peak multipath based error of the order of 1 cm. in altitude or lateral distance. Also of major significance is the desirability of an antenna having a unitary phase center of accurately determined position, to permit precision determinations of phase of received signals and avoid introduction of phase discrepancies. Antenna systems having the desired characteristics in this regard are described in applicant's U.S. Pat. No. 5,534,882 (the '882 patent) which is hereby incorporated herein by reference.
Antennas providing a circular polarization characteristic in all directions horizontally and upward from the horizon (i.e., omnidirectional coverage from the zenith to the horizon) with a sharp pattern cut off at the horizon are described in applicant's U.S. Pat. No. 6,201,510 (the '510 patent) which is hereby incorporated herein by reference. Antennas described in the '510 patent may be employed to provide up to 30 dB suppression of reflected GPS multipath signals.
Objects of the present invention are to provide new and improved antenna systems, including antenna systems capable of receiving GPS signals with very low susceptibility to effects of reflected ground multipath GPS signals.